Liquid crystal display (LCD) devices are widely used in projection TVs or as a video monitor for a computer system nowadays. They have become an essential part of a modern lap-top or notebook computer because of their compact size and light weight as compared to conventional display devices using cathode ray tubes. How to achieve a wider viewing angle and lower color dispersion for a liquid crystal display has been an important area that most manufacturers dedicate significant effort to.
Conventional LCD devices have a known problem related to color difference. When a liquid crystal cell of an LCD is applied a voltage, its electro-optical (E-O) characteristic is different with respect to the different optical wavelengths of red, blue and green lights. The color dispersion or birefringence dispersion caused by the difference in the electro-optical characteristic results in color difference in the LCD. For a twisted nematic (TN) mode LCD, the color difference has not been a very disturbing problem.
In recent years, the electrically controlled birefringence (ECB) effect is increasingly used in many LCD devices for obtaining a wide viewing angle. These types of liquid crystal cells include optically compensated bend (OCB) liquid crystal, closed-cavity liquid crystal (CCLC), vertically alignment (VA) mode liquid crystal, and many others. The color dispersion in these LCD devices can be very pronounced because the transimittance T of the liquid crystal is described as EQU T=sin.sup.2 [d .DELTA.n.sub.eff (V)/.lambda.],
where d is the thickness of the liquid crystal and .DELTA.n.sub.eff is the effective difference in the refractive index, V is the applied electrical voltage, and .lambda. is the wavelength. As .lambda. decreases, .DELTA.n.sub.eff increases. Therefore, the red, green, and blue colors are unbalanced. FIG. 1 shows typical transmission curves as a function of an applied voltage for different wavelengths. The color dispersion becomes a significant drawback for the LCD.
Seiichi Nagata et al. of Matsushita Electric Industrial Co., Ltd, of Japan presented a Twisted-Nematic Liquid Crystal Full-Color Display Panel with Reduced Rotatory Dispersion" in SID 85 DIGEST, for overcoming the drawback of LCD color dispersion by means of multiple cell gaps. The technique of Seiichi Nagata et al. requires an extra process in manufacturing the color filter. It also has a poor cell gap uniformity.
T. Konno et al. of Tohoku University of Japan presented a paper titled OCB-cell Using Polymer Stabilized Bend Alignment" in ASIA DISPLAY '95 for improving the switching speed from a splay alignment to a bend alignment of the liquid crystal cell when the display is turned on. The technique of T. Konno et al. induces an extended polymer network in the direction of liquid crystal alignment in order to stabilize and maintain the bend alignment of the liquid crystal even after the applied voltage is turned off. The approach presents a solution to achieve fast response for an OCB-mode LCD. However, the color dispersion issue has not been addressed.